As the owner of a 1988 Sierra Cosworth I would like to put down a vote for the YBB to YBP 2.0 Cosworth engine.
This engine can be modified to give over 500HP. Most common output when tuned is in the region of 270-350HP
Sorry about all the pics...

A few photos of the engine(taken from
www.grahamgoode.com )
Here`s a Ford press photo of my car(same colour etc)
Here are a select few that has been modified(taken from various sites)
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Arne
Ford Sierra Cosworth 2wd
Stage1 270hp @ 1.1bar